Apparatus Having a Photonic Crystal

ABSTRACT

An apparatus, including a substrate, where at least a portion of the substrate has a convex surface, and a photonic crystal disposed over the convex surface. The photonic crystal is substantially conformal to at least a portion of the convex surface.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application and claims the benefit andpriority of U.S. patent application Ser. No. 11/046,586 filed Jan. 28,2005.

BACKGROUND Description of the Art

As the demand for cheaper and higher performance electronic devicescontinues to increase there is a growing need to develop higher yieldlower cost manufacturing processes for electronic devices especially inthe area of optical devices. In particular there is a demand for higherperformance as well as improved efficiency in lighting technology.

Although incandescent lamps are inexpensive and the most widely utilizedlighting technology in use today, they are also the most inefficientlighting source in regards to the amount of light generated per unit ofenergy consumed. An incandescent lamp works by heating a filament,typically tungsten, to a very high temperature so that it radiates inthe visible portion of the electromagnetic spectrum. Unfortunately, atsuch high temperatures the filament radiates a considerable amount ofenergy in the non-visible infrared region of the electromagneticspectrum.

If these problems persist, the continued growth and advancements in theuse of opto-electronic devices, especially in the area of photoniccrystals, in various electronic products, will be reduced. In areas likeconsumer electronics, the demand for cheaper, smaller, more reliable,and higher performance electronics constantly puts pressure on improvingand optimizing performance of ever more complex and integrated devices.The ability to optimize lighting performance efficiency will open up awide variety of applications that are currently either impractical, orare not cost effective.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a perspective view of a portion of a substrate havingspheres disposed thereon according to an embodiment of the presentinvention.

FIG. 1 b is a cross-sectional view of a portion of the substrate shownin FIG. 1 a.

FIG. 1 c is a cross-sectional view of a portion of a substrate accordingto an alternate embodiment of the present invention.

FIG. 2 a is a perspective view of a colloidal crystal formed on acylindrically shaped substrate according to an alternate embodiment ofthe present invention.

FIG. 2 b is a cross-sectional view along 2 b-2 b of the colloidalcrystal shown in FIG. 2 a.

FIG. 3 a is a perspective view of a colloidal crystal formed on theouter surface of a tubular shaped substrate according to an alternateembodiment of the present invention.

FIG. 3 b is a cross-sectional view along 3 b-3 b of the colloidalcrystal shown in FIG. 3 a.

FIG. 3 c is a cross-sectional view of a colloidal crystal according toan alternate embodiment of the present invention.

FIG. 4 is a perspective view of an incandescent source according to anembodiment of the present invention.

FIG. 5 is a perspective view of an incandescent source according to analternate embodiment of the present invention.

FIG. 6 a is a perspective view of a portion of a colloidal crystalaccording to an embodiment of the present invention.

FIG. 6 b is a perspective view of a portion of an inverse opal crystalaccording to an alternate embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is directed to various photonic structures utilizingcolloidal crystals. The present invention includes a wide variety ofphotonic structures formed on, over or both on and over curved surfacesincluding, for example, wires and fiber optic cables. Photonic crystals,typically, are spatially periodic structures having usefulelectromagnetic wave properties, such as photonic band gaps. Photoniccrystals, for example, having the proper lattice spacing, offer thepotential of improving the luminous efficacy of an incandescent lamp bymodifying the emissivity of the tungsten filament. Such a filament,incorporated into a photonic crystal or encircled or surrounded by aphotonic crystal, would emit a substantial fraction of its radiation inthe visible portion of the spectrum and little or no light in thenon-visible portions such as the infrared portion of the electromagneticspectrum. Since many filaments, including spirally wound filaments,utilized as incandescent sources have a large degree of cylindricalsymmetry the ability to form photonic crystals on curved surfacesprovides for simpler manufacturing processes to make incandescent lightsources, having a lower cost, and a higher luminous efficiency. Inaddition, such a colloidal crystal may also be formed on an opticalfiber to deter loss of light at desired wavelengths.

It should be noted that the drawings are not true to scale. Further,various elements have not been drawn to scale. Certain dimensions havebeen exaggerated in relation to other dimensions in order to provide aclearer illustration and understanding of the present invention. Inparticular, vertical and horizontal scales may differ and may vary fromone drawing to another. In addition, although some of the embodimentsillustrated herein are shown in two dimensional views with variousregions having height and width, it should be clearly understood thatthese regions are illustrations of only a portion of a device that isactually a three dimensional structure. Accordingly, these regions willhave three dimensions, including length, width, and height, whenfabricated on an actual device.

Moreover, while the present invention is illustrated by variousembodiments, it is not intended that these illustrations be a limitationon the scope or applicability of the present invention. Further, it isnot intended that the embodiments of the present invention be limited tothe physical structures illustrated. These structures are included todemonstrate the utility and application of the present invention topresently preferred embodiments.

An embodiment of apparatus 100 employing the present invention isillustrated, in a perspective view, in FIG. 1 a. In this embodiment,apparatus 100 includes substrate 120 that includes at least a portion ofthe substrate forming convex surface 112 over which photonic crystal 102is disposed. In addition, the photonic crystal is formed substantiallyconformal to convex surface 112 as illustrated in the cross-sectionalview along 1 b-1 b in FIG. 1 b. In this embodiment, spheres 122 may bedisposed on any substrate having essentially a convex surface. Examplesof such substrate structures include, but are not limiting as to thenature of the present invention, rod-like substrates, cylindricallyshaped substrates, tubularly shaped substrates, conically shapedsubstrates, and substrates having a closed surface such as the outersurface of a sphere. In still other embodiments, various layers such asan adhesive layer or other layer having particular optical or dielectricproperties may be disposed between substrate 120 and photonic crystal102. Photonic crystal 102, as illustrated in FIGS. 1 a-1 c is what iscommonly referred to as a colloidal crystal or opaline crystallinearray. The colloidal crystal is formed utilizing spheres 122. Inalternate embodiments, photonic crystal 102 may also form what iscommonly referred to as an inverse opal structure where interstitialvolume 124 between the spheres is infiltrated and filled with a secondmaterial with the optional subsequent removal of spheres 122. Typically,the optional removal of the spheres after infiltration is completed willdepend on whether the interstitial material has a higher refractiveindex than the spheres. In those cases where it is higher then thespheres need not, but may, still be removed. Generally, photonic crystal102 will be formed utilizing multiple layers of spheres having typicallya close-packed geometry, as illustrated in a cross sectional view inFIG. 1 c, forming a face centered cubic crystalline structure (FCC), ahexagonal close packed structure (HCP), or other randomly stackedpolycrystalline structure with each sphere predominantly touching sixother spheres in one layer. However, in alternate embodiments otherstructures also may be utilized including, for example, simple cubic,body centered cubic and tetragonal packing. Further, in someembodiments, a single layer of spheres may be desirable. In thoseembodiments, utilizing multiple layers photonic crystal 102′, as shownin FIG. 1 c, may also form a photonic band gap crystal. Substrate 120,in this embodiment, may be formed from any material that has the desiredoptical, chemical, and mechanical properties for utilization inapparatus 100. For example, in one embodiment, substrate 120 may beformed from various glasses for those applications desiring substantialtransparency in the visible portion of the electromagnetic spectrum. Ina second embodiment, substrate 120 may be a metal wire such as tungstenor a tungsten alloy for those applications desiring substantial emissionin the infrared or visible portion of the electromagnetic spectrum (e.g.an incandescent source heated to a high temperature). Any metal or alloymay be utilized the particular material chosen will depend on theparticular portion of the spectrum to be used and the desired intensity.In still other embodiments, substrate 120 may be a fiber in, forexample, a fiber optic application or substrate 120 may be utilized asan optical component such as a rod lens. Such fibers and opticalcomponents may be formed from various glasses, polymers or any otherappropriate material having the desired optical properties for theparticular application in which it will be utilized. Spheres 122, inthis embodiment, may be formed from any material that is formable intospheres and provides the desired dielectric constant for the particularapplication in which the photonic crystal is utilized. The size of thespheres generally ranges from a few microns in diameter to a fewnanometers in diameter. Both the particular material utilized to formspheres 122 and the size of the spheres will depend on the particularoptical properties of the photonic crystal utilized in apparatus 100.For example, silica or polymer spheres may be utilized in thoseapplications desiring a reduction in light lost as it propagates alongan optical fiber. Another example is the use of metal spheres to formhigh temperature filaments for emitting light in the infrared and/orvisible portions of the electromagnetic spectrum. Still another exampleis to use spheres having a differential solubility over an infiltrationmaterial to form inverse opal structures such as silica spheres removedby hydrofluoric acid in a tungsten inverse opal structure. Further, thephotonic crystal may be formed utilizing spheres having different sizes.A wide variety of combinations of different sphere sizes may be used inthe present invention. For example, each successive layer of spheres mayincrease or decrease in size, or the size of spheres may alternate insuccessive layers or every nth layer may vary or an alternating group oflayers may be varied. In addition, spheres of different sizes also maybe utilized to form a single layer such as in the formation of a binary(AB₂) colloidal crystal.

An alternate embodiment of the present invention is shown in aperspective view in FIG. 2 a. In this embodiment, apparatus 200 includessubstrate 220 having generally a cylindrically shaped outer surface.However, in alternate embodiments, substrate 220 may have any curvedshape forming a substantially rod-like substrate. Substrate 220 includesmultiple layers of spheres 222 disposed on the outer or external surfaceof substrate 220 as illustrated in a cross-sectional view in FIG. 2 b;however, in alternate embodiments, a single layer of spheres 222 alsomay be utilized. In this embodiment, the spheres form photonic crystal202; however, in alternate embodiments, photonic crystal 202 may beformed utilizing an inverse opal structure where interstitial volume 224between the spheres is infiltrated and filled with a second materialwith the optional subsequent removal of spheres 222. In one particularembodiment, photonic crystal 202 forms a photonic band gap crystalincluding inverse opal band gap structures. In still other embodiments,various layers such as an adhesive layer or other layer havingparticular optical or dielectric properties may be disposed betweensubstrate 220 and photonic crystal 202. In this embodiment, photoniccrystal 202 is a coaxial, colloidal crystal tuned to yield a band gap ina desired spectral region. In addition in this embodiment, photoniccrystal 202 fully encloses and/or encircles the outer surface ofsubstrate 220. For example, in those embodiments, utilizing a metalwire, such as tungsten, the desired spectral region may be in theinfrared or visible portions of the electromagnetic spectrum. In oneembodiment, substrate 220 may be a tungsten wire with a tungsten inverseopal structure disposed on the outer surface of substrate 220 forming anincandescent filament. In addition, the wire may be formed into variousshapes, such as a spiral shape. In a second embodiment, substrate 220also may be a tungsten wire with tungsten spheres or other metal with alow vapor pressure at high temperatures forming the colloidal crystal.In still other embodiments, substrate 220 may be an optical fiber wherephotonic crystal 202 is tuned to reduce the amount of light lost in theoptical fiber during use, or substrate 220 may be a lens such as a rodlens.

An alternate embodiment of the present invention is shown in aperspective view in FIG. 3 a. In this embodiment, apparatus 300 includessubstrate 320 having generally a cylindrically shaped tubular structure.However, in alternate embodiments, substrate 320 may have any curvedshape forming essentially a tubular-like structure. Substrate 320includes multiple layers of spheres 322 disposed on the outer orexternal surface of substrate 320 as illustrated in a cross-sectionalview, in FIG. 3 b. In this embodiment, the spheres form photonic crystal302; however, in alternate embodiments, photonic crystal 302 may beformed utilizing an inverse opal structure. In one particular embodimentphotonic crystal 302 forms a photonic band gap crystal including inverseopal band gap structures. In still other embodiments, various layerssuch as an adhesive layer or other layer having particular optical ordielectric properties may be disposed between substrate 320 and photoniccrystal 302. In this embodiment, photonic crystal 302 is a colloidalcrystal tuned to yield a band gap in a desired spectral region. Analternate embodiment is illustrated in FIG. 3 c where substrate 320includes multiple layers of spheres 322 disposed on both the externalsurface and the inner or internal surface of substrate 320 to formphotonic crystals 302 and 302′. Photonic crystals 302 and 302′illustrated in FIG. 3 c may also include inverse opal structures as wellas combinations of a colloidal crystal and an inverse opal structure. Inaddition, the photonic crystals may be formed utilizing spheres havingdifferent sizes as previously described for the embodiments shown inFIGS. 1 a-1 c.

An alternate embodiment of the present invention is shown in aperspective view in FIG. 4. In this embodiment, apparatus 400 includesfilament 430 disposed within, and substantially coaxial with, substrate420 which has a cylindrically shaped tubular structure. Substrate 420includes multiple layers of spheres 422 disposed on the outer orexternal surface of substrate 420; however, in alternate embodiments asingle layer of spheres may be utilized. However, in an alternateembodiment, spheres 422 may be disposed on both the external surface andthe inner or internal surface of substrate 420 to form multiple photoniccrystals. In this embodiment, substrate 420 is sufficiently transparentto provide the desired optical performance; however, in alternateembodiments, substrate 420 may be removed after the formation of thecolloidal crystal, such as by etching, so that the optical properties ofthe substrate would not be important. In this embodiment, the spheresform photonic crystal 402; however, in alternate embodiments, photoniccrystal 402 may be formed utilizing an inverse opal structure. In oneparticular embodiment photonic crystal 402 forms a photonic band gapcrystal including inverse opal band gap structures where the photoniccrystal is tuned to yield a band gap in a desired spectral region in theinfrared or visible region of the electromagnetic spectrum asrepresented by arrows 410. In one embodiment, filament 430 is a tungstenwire and photonic crystal 402 is tuned to pass visible light providingfor an incandescent source having higher efficiency compared toconventional incandescent sources. In alternate embodiments, filament430 may be formed from other metals, including other refractory metalssuch as Ta, Mo, and Re, or cermets. In addition, photonic crystal 402,may, for example, be tuned to pass infrared radiation in a desiredregion. Again providing higher efficiency compared to conventionalsources. In still another embodiment of the present invention apparatus500 includes spiral filament 530 disposed within and substantiallycoaxial with substrate 520 as illustrated in a perspective view in FIG.5. Substrate 520 includes multiple layers of spheres 522 disposed on theouter surface of substrate forming photonic crystal 502; however inalternate embodiments, photonic crystal 502 may also be formed utilizinga single layer of spheres or an inverse opal structure. As describedpreviously for the embodiment shown in FIG. 4 the combination of spiralfilament 530 and photonic crystal 502 generally provides for moreefficient infrared and visible light sources as represented by arrows510. In still another embodiment, a photonic crystal may be formeddirectly on the outside surface of the coiled filament where thephotonic crystal formed on the filament and the photonic crystal formedon the tubular structure are optimized to provide a more efficient lightsource.

The colloidal crystals shown in FIGS. 1 -5 may be formed by a variety oftechniques. For example, sedimentation, and evaporation may be utilizedto deposit monolayer and multilayer spheres on a substrate. Twoexemplary techniques have been used to form multilayer spheres on convexsurfaces. The substrate is suspended and/or immersed in a solution sothat the longitudinal axis of the substrate is essentially perpendicularto the meniscus formed by the solution. The solution includes a mixtureof spheres and a solvent. For example the solution may include silicaspheres or polymeric spheres, such as polystyrene, suspended in anethanol solvent. Generally, the volume fraction of spheres is in therange from about 1 percent to about 10 percent. A wide variety ofsolvents may be utilized such as water, ethanol, methanol, propanol, andhexanes. After suspending and/or immersing the substrate in the solutionthe solution is allowed to evaporate. Depending on the size of spheresand the material utilized to form the spheres the evaporation may becarried out anywhere from room temperature up to just below the boilingpoint of the solvent. For example, for silica spheres having a diameterless than about 500 nanometers the solution may be evaporated at or nearroom temperature, whereas for silica spheres having a diameter greaterthan about 500 nanometers the solution may be evaporated at or near itsboiling point. Generally, when the solution is heated above roomtemperature the vessel holding the solution is enclosed and partiallysealed so that the solution may evaporate in a controlled manner andconvection currents in the solvent substantially hinder the spheres fromsettling. The thickness or number of layers of spheres deposited may becontrolled by varying the speed of evaporation, the volume fraction ofspheres in suspension, or combinations of both. In addition, thickercolloidal crystals also may be formed by carrying out multipledeposition cycles. To hinder the peeling off or partial redispersion ofthe previously deposited films during subsequent depositions it has beenfound to be advantageous to sinter the spheres. For example, in thoseembodiments utilizing silica spheres sintering may be carried oututilizing tetramethyl orthosilicate for several minutes at about 80° C.Another example is to heat silica spheres to about 600° C. to improvethe structural integrity of the colloidal crystal without utilizing asintering agent. In still other embodiments, other sintering agents,times, and temperatures also may be utilized. The particular, sinteringagent, time, and temperature is application specific because sinteringmay affect the filling factor or optical properties of the photoniccrystal or various combinations of both. In addition, multilayercolloidal crystals having different colloidal sphere sizes may be formedutilizing multiple depositions. For example, AB, ABA, ABC multilayercrystals may be formed where the letters A, B, and C each represent atleast one layer of spheres having a different sphere diameter from theother letters. In still other embodiments, multiple sized spheres alsomay be utilized in a single solution to generate, for example, binaryAB₂ crystal structures. Further, the spheres of different sizes may beformed utilizing different materials have different dielectric constantsgenerating a colloidal crystal having a spatially varying dielectricconstant. It may also be desirable, depending on the particularapplication in which the photonic crystal will be utilized, to generateor create portions of the substrate surface free of the spheres. In suchembodiments, patterning of the substrate may be generated by selectivelyapplying a sacrificial layer in those areas where it is desired that thespheres do not form. For example, in the embodiment shown in FIG. 4, theinternal surface of substrate 420 may be coated or filled with apolyamic acid solution which may be removed at a later time utilizing asolvent such as N,N-dimethylacetamide (DMAC) or N-methyl-2-pyrrolidone(NMP), or a strong basic solution such as potassium hydroxide (KOH). Awide variety of inorganic or organic sacrificial materials may beutilized. The particular material chosen will depend on various factorssuch as the particular spheres, solvent, and temperature utilized toform the colloidal crystal.

For those embodiments utilizing an inverse opal crystal structure avariety of deposition techniques may be utilized to fill theinterstitial volume formed between the spheres such as atomic layerdeposition (ALD), chemical vapor deposition (CVD), electro-deposition,and electroless deposition and other wet infiltration methods. Anexemplary technique utilizes atomic layer deposition to fill orinfiltrate the interstitial volume of the colloidal crystal. In oneembodiment a tungsten inverse opal structure may be generated utilizingalternating exposures of the colloidal crystal to tungsten hexafluoride(WF₆) and silicon hydride (e.g. SiH₄, Si₂H₆, Si₃H₈ and mixtures ofvarious silicon hydrides). The tungsten film growth may be achievedutilizing an alternating sequence of exposures of WF₆ and Si₂H₆ in thetemperature range from about 100° C. to about 400° C. It is believedthat the disilane reactant serves a sacrificial role to strip fluorinefrom tungsten limiting the incorporation of silicon into the film;however, the present invention is not limited to such a mechanism. Otherchemistries also may be utilized such as tungsten hexacarbonyl as atungsten precursor material and boron compounds such as a boron hydrideas a reducing agent. In alternate embodiments, other silicon hydridesalso may be utilized. In still other embodiments a wide range ofinorganic materials also may be utilized. Tungsten nitride, titaniumdioxide, graphite, diamond, tungsten carbide, hafnium carbide, andindium phosphide are just a few examples. After the interstitial volumein the crystal is filled or substantially filled the silica spheres maythen be removed by soaking in a aqueous hydrofluoric acid solution (i.e.typically about 2 weight percent) to form inverse opal photonic crystal604 as illustrated in FIG. 6 b. FIGS. 6 a and 6 b illustrate thedifferences between a colloidal crystal and an inverse opal crystal.FIG. 6 a represents a portion of colloidal crystal 604 which has aclose-packed geometry; whether the structure is face-centered cubic,hexagonal close-packed or randomly stacked with each sphere 626 touchingsix other spheres in one layer. Interstitial volume 624 is the volume ofthe crystal not occupied by spheres 626. FIG. 6 b represents a portionof an inverse opal photonic crystal 604′ where interstitial volume 624has been infiltrated or filled with an inorganic material and spheres626 have been removed. The particular inorganic material utilized willdepend on the particular application in which the photonic crystal isutilized. ALD provides an exemplary technique for thin film depositionin deep structures, complex structures, or both. In addition, ALD alsoprovides control in the chemical composition of the deposited film byselection of various precursors, various deposition temperatures andpressures, and combinations of these parameters. Further, the generallylow deposition rates (i.e. typically on the order of a few tenths of ananometer per cycle) allows for a more uniform growth rate and moreuniform thickness control in the narrow voids formed in the colloidalcrystal providing a cost-effective process to fabricating photonic bandgap structures.

1. A method of using a photonic crystal, comprising heating anincandescent filament disposed within a tubularly-shaped photoniccrystal substantially encircling said incandescent filament.
 2. A methodof using a photonic crystal, comprising: generating a portion of theelectromagnetic spectrum from an incandescent filament; and transmittingsaid portion of the electromagnetic spectrum through at least a portionof a photonic crystal disposed over a convex surface formed on at leasta portion of a substrate, wherein said photonic crystal is substantiallyconformal to at least a portion of said convex surface.
 3. The method inaccordance with claim 2, wherein transmitting said portion of theelectromagnetic spectrum further comprises transmitting said portion ofthe electromagnetic spectrum radially through at least a portion of atubularly-shaped photonic crystal, wherein at least a portion of saidincandescent filament is disposed within said tubularly-shaped photoniccrystal.
 4. The method in accordance with claim 2, wherein generatingsaid portion of the electromagnetic spectrum from said incandescentfilament further comprises generating said portion of theelectromagnetic spectrum from a spirally wound filament.
 5. The methodin accordance with claim 4, wherein generating said portion of theelectromagnetic spectrum from said spirally wound filament furthercomprises generating said portion of the electromagnetic spectrum fromsaid spirally wound filament having a filament photonic crystalconformal to and disposed on at least a portion of said spirally woundfilament.
 6. The method in accordance with claim 2, wherein generatingsaid portion of the electromagnetic spectrum from said incandescentfilament further comprises generating said portion of theelectromagnetic spectrum from a filament photonic crystal conformal toand disposed on at least a portion of said incandescent filament
 7. Themethod in accordance with claim 2, wherein transmitting said portion ofthe electromagnetic spectrum further comprises transmitting said portionof the electromagnetic spectrum through at least a portion of acolloidal photonic crystal.
 8. The method in accordance with claim 2,wherein transmitting said portion of the electromagnetic spectrumfurther comprises transmitting said portion of the electromagneticspectrum through at least a portion of an inverse opal photonic crystal.9. The method in accordance with claim 2, wherein transmitting saidportion of the electromagnetic spectrum further comprises transmittingsaid portion of the electromagnetic spectrum through at least a portionof a photonic band gap crystal.
 10. The method in accordance with claim2, wherein transmitting said portion of the electromagnetic spectrumfurther comprises transmitting said portion of the electromagneticspectrum through at least a portion of a spherically-shaped photoniccrystal.
 11. The apparatus in accordance with claim 2, whereintransmitting said portion of the electromagnetic spectrum furthercomprises transmitting said portion of the electromagnetic spectrumthrough at least a portion of said photonic crystal having a spatiallyperiodic structure.